internal isochron - This is an isochron determined by analyses of mutliple minerals within a particular sample. Examples include Fig. 1 of Lugmair & Shukolyukov. If the data for various minerals lie along along a single line in an isotope plot such as Fig. 1, it is taken to be good evidence for a real age of a particular sample (e.g., the time of crystallization for individual rocks in Fig. 1). This contrasts with a model age, which involves an additional (model) assumption of some sort. For example, Fig. 2 of Lugmair & Shukolyukov assumes that various whole-rock HED meteorites (e.g., the diogenites Shalka [SHA] and Johnstown [JT], and the remaining eucrites), are all related. This is probably a good assumption, but the isochron in this case is based on a model, and refers to a different event (in this case interpreted as the time when initial differentiation of the parent body to produce eucrites and diogenites occurred).
primitive achondrite - This is an achondrite that has igneous textures but more-or-less chondritic mineralogy. Examples include the acapulcoites, winonaites, lodranites, and silicate inclusions in IAB irons. Some people lump the brachinites in this category, too.
angrite - This is a type of achondrite that features
prominently in efforts to link data for short-lived to long-lived radionuclides,
as evidenced in the reading. Angrites are extraterrestrial basalts
and contain mainly plagioclase and clinopyroxene, and smaller amounts of
olivine and kirschtenite (Ca-Fe-Mg olivine). They differ from eucrites,
which are also achondrite basalts, in terms of mineral chemistry, mineralogy
(e.g., kirschtenite in angrites, silica minerals in eucrites), texture,
shock and metamorphic history, and age. One of the last angrites
to be discovered, D'Orbigny, has vesicles, as seen below.
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Two images showing the interior of the D'Orbigny angrite,
showing plagioclase (light grey), pyroxene (grey to brown-red), olivine
(pale green), and vesicles (pits). Vesicles are rare in basaltic
achondrites; their presence here implies that D'Orbigny formed near the
surface of its parent body.
non-cumulate vs cumulate eucrite - There
are a variety of eucrite achondrites. Non-cumulate eucrites
are finer-grained and have textures which suggest they formed either as
basalt flows, dikes, sills, or possibly impact melts; cumulate eucrites
are coarser-grained and appear to have formed by crystal settling in a
magma chamber.